User friendly is pretty vague but if we're talking about the average user who uses the computer to browse the web, play some games on Steam with friends and do some document editing that is all covered. Linux Mint would be an excellent option. The installation is arguably easier than Windows (because you won't get all those pesky telemetry and data gathering questions), visually it looks and functions similarly to Windows, most standard software is handled by a package manager so no using terminal to install things and the distro prefers stability and ease of access over fancy bells and whistles.
If there are any user issues they're most likely because of third party developer not properly supporting Linux and there's nothing Linux can do. One example is anti-cheat for games. You can't play Riot games on Linux simply because their anti-cheat won't allow you to play their games on Linux and there's nothing Linux devs can do about it.
I don't know when was the last time you used Linux but I'd say the user experience has definitely improved in the last 10 years. It's gotten to a point where I'm actually willing to recommend Linux to users whose needs I'm familiar with and know they not going to have some weird needs that need the use of a terminal.
True, but the same issue happens with reddit as well, for example gaming vs games vs truegaming. Over time those communities either found their niche (gaming sub became mostly memes, games sub became news and discussions and truegaming tried to become a more serious discussing sub). Actually there were way more gaming subs but unless they found their niche they died out. So people gravitating towards specific communities is a natural occurrence.
As for trying to automatically consolidate communities across instances, it sounds like a great idea on paper but seems like technical she moderation headache, because you won't have a clear source of truth. Let's say instance A and instance B both have a community called news. The same news article with the same title is posted on both communities on both instances by different users. Assuming we want to consolidate those posts into one, which instance post will be shown or in more technical terms, which instance becomes the source of truth for that post? Who makes that decision? What if there's also instance C with the same community and the same post but that instance isn't federated with instance A, how do we consolidate posts? Each community has its own moderators and moderation rules, who is allowed to moderate the post? What if the moderation rules contradict between instances and both instances want to apply the rules independently, are they supposed to split the post?
Maybe there is an elegant solution to all the problems but I don't see there being one. I'm not against the idea, the problem is you want to solve its something I have given some thought and because of that I just don't see it working out the way you're imagining it.